Thin film transistors (TFTs) are widely used as switching elements in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. The thin film transistor (TFT) is an example of a field effect transistor (FET). The best-known example of an FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today's conventional switching element for high-speed applications. For applications in which a transistor needs to be applied to a substrate, a thin film transistor is typically used. A critical step in fabricating the thin film transistor involves the deposition of a semiconductor onto the substrate. Presently, most thin film devices are made using vacuum deposited amorphous silicon as the semiconductor.
Amorphous silicon as a semiconductor for use in TFTs still has its drawbacks. The deposition of amorphous silicon, during the manufacture of transistors, requires relatively difficult or complicated processes such as plasma enhanced chemical vapor deposition and high temperatures (typically about 360° C.) to achieve the electrical characteristics sufficient for display applications. Such high processing temperatures disallow deposition on substrates made of certain plastics that might otherwise be desirable for use in applications such as flexible displays.
There is a growing interest in depositing thin film semiconductors on plastic or flexible substrates, particularly because these supports would be more mechanically robust, lighter weight, and potentially lead to cheaper manufacturing by allowing roll-to-roll processing. A useful example of a flexible substrate is polyethylene terephthalate. Such plastics, however, limit device processing to below 200° C.
There is also interest in utilizing processes for deposition that do not involve the expense associated with vacuum processing. In typical vacuum processing, a large metal chamber and sophisticated vacuum pumping systems are required in order to provide the necessary environment. These items increase the capital cost of systems and preclude the easy use of continuous web based systems.
In the past decade, various materials have received attention as a potential alternative to amorphous silicon for use in semiconductor channels of thin film transistors. Semiconductor materials are desirable that are simpler to process, especially those that are capable of being applied to large areas by relatively simple processes. Semiconductor materials that can be deposited at lower temperatures would open up a wider range of substrate materials, including plastics, for flexible electronic devices.
Thus, thin film transistors made of easily deposited semiconductor materials can be viewed as a potential key technology for circuitry in various electronic devices or components such as display backplanes, portable computers, pagers, memory elements in transaction cards, and identification tags, where ease of fabrication, mechanical flexibility, and/or moderate operating temperatures are important considerations.
The discovery of practical inorganic semiconductors as a replacement for current silicon-based technologies has also been the subject of considerable research efforts. For example, metal oxide semiconductors are known that constitute zinc oxide, indium oxide, gallium indium zinc oxide, tin oxide, or cadmium oxide deposited with or without additional doping elements including metals such as aluminum. Such semiconductor materials, which are transparent, can have an additional advantage for certain applications, as discussed below.
A semiconductor material useful in a TFT must display several characteristics. In typical applications of a thin film transistor, the desire is for a switch that can control the flow of current through the device. As such, it is desired that when the switch is turned on a high current can flow through the device. The extent of current flow is related to the semiconductor charge carrier mobility. When the device is turned off, it is desired that the current flow be very small. This is related to the native charge carrier concentration. Furthermore, it is desired that the device be weakly or not at all influenced by visible light to avoid a light-protection layer. In order for this to be true, the semiconductor band gap must be sufficiently large (>3 eV) so that exposure to visible light does not cause an inter-band transition. Zinc oxide based materials are capable of delivering these features. Furthermore, in a real high volume web based atmospheric manufacturing scheme, it is highly desirable that the chemistries used in the process be both cheap and of low toxicity, which can be satisfied also by the use of ZnO-based materials and the majority of its precursors.
High on/off ratios result when the device in its off state has very low current flow, often referred to as current leakage. There are many applications in which low leakage is essential. In display applications, low leakage is required for the pixel select transistor. This select transistor is a switch that allows a charge to enter in and be stored in the pixel. In a perfect transistor without leakage, once the charge is stored in the pixel the transistor is switched to its off state and this charge cannot diminish by leakage through the select transistor. Too high of an off current in a transistor will cause a decay in stored charge in the pixel which results in poor display performance.
Another transistor characteristic that is relevant for useful operation is the steepness of the transistor turn on, represented by the subthreshold slope. As the gate voltage of a transistor is varied, the transistor will start in an off state, characterized by low current flow, and transition to an on state, characterized by high current flow. When the gate voltage reaches a point at which the transistor begins to turn on, there is a substantial increase in drain current with increasing gate voltage. This increase, called the subthreshold slope, is measured in volts of gate voltage per decade of drain current. This expression therefore represents the number of volts of gate voltage required to produce a 10-fold increase in drain current. Lower values of the subthreshold slope indicate faster device turn on and are desirable.
Various processes for making zinc oxide films have been disclosed, both high temperature and low temperature processes, including radio frequency magnetron sputtering or modified reactive planar magnetron sputtering.
Ohya et al. (Japanese Journal of Applied Physics, Part 1, January 2001, vol. 40, no. 1, pages 297-8) disclose a thin film transistor of ZnO fabricated by chemical solution deposition.
Transparent conducting oxides are reviewed in the August 2000 issue of the Materials Research Bulletin, Volume 25 (8) 2000, devoted to materials and properties of transparent conducting oxide compounds.
One low temperature process for deposition of such oxide semiconductors is disclosed in US 2004/0127038 to Carcia et al. This publication discloses a semiconductor deposition process that uses magnetron sputtering of a metal oxide (ZnO, In2O3, SnO2, CdO) or metal (Zn, In, Sn, Cd) target in an atmosphere with a controlled partial pressure of oxygen in an inert gas. This is a low temperature process that is compatible with temperature sensitive substrates and components, for example, drive circuits for displays on flexible, polymer substrates. The field effect transistors of Carcia et al. are based on a nominally undoped metal oxide semiconductor that must be deposited using physical vapor deposition or chemical vapor deposition, preferably rf (radio frequency) magnetron sputtering.
Steven K. Volkman et al., in “A novel transparent air-stable printable n-type semiconductor technology using ZnO nanoparticles,” 2004 IEEE International Electron Device meeting Technical Digest, pp. 769, 2004, disclose a method for producing thin film transistors using organically stabilized zinc-oxide nanoparticles. The disclosed process involves an exposure to a temperature of 400° C.
Although successful zinc oxide based devices have been made with sputtering techniques, it is clear that very precise control over the reactive gas composition (such as oxygen content) is required to produce good quality devices.
Chemical vapor deposition (CVD) techniques, in which two reactive gasses are mixed to form the desired film material, can be useful routes to achieving high quality film growth. Atomic layer deposition (“ALD”) is yet another alternative film deposition technology that can provide improved thickness resolution and conformal capabilities, compared to its CVD predecessor. The ALD process segments the conventional thin-film deposition process of conventional CVD into single atomic-layer deposition steps.
ALD can be used as a fabrication step for forming a number of types of thin-film electronic devices, including semiconductor devices and supporting electronic components such as resistors and capacitors, insulators, bus lines, and other conductive structures. ALD is particularly suited for forming thin layers of metal oxides in the components of electronic devices. General classes of functional materials that can be deposited with ALD include conductors, dielectrics or insulators, and semiconductors.
Examples of useful semiconducting materials are compound semiconductors such as gallium arsenide, gallium nitride, cadmium sulfide, zinc oxide, and zinc sulfide.
A number of device structures can be made with the functional layers described above. A capacitor results from placing a dielectric between two conductors. A diode results from placing two semiconductors of complementary carrier type between two conducting electrodes. There may also be disposed between the semiconductors of complementary carrier type a semiconductor region that is intrinsic, indicating that that region has low numbers of free charge carriers. A diode may also be constructed by placing a single semiconductor between two conductors, where one of the conductor/semiconductors interfaces produces a Schottky barrier that impedes current flow strongly in one direction. A transistor results from placing upon a conductor (the gate) an insulating layer followed by a semiconducting layer. If two or more additional conductor electrodes (source and drain) are placed spaced apart in contact with the top semiconductor layer, a transistor can be formed. Any of the above devices can be created in various configurations as long as the critical interfaces are created.
Advantageously, ALD steps are self-terminating and can deposit precisely one atomic layer when conducted up to or beyond self-termination exposure times. An atomic layer typically ranges from about 0.1 to about 0.5 molecular monolayers, with typical dimensions on the order of no more than a few Angstroms. In ALD, deposition of an atomic layer is the outcome of a chemical reaction between a reactive molecular precursor and the substrate. In each separate ALD reaction-deposition step, the net reaction deposits the desired atomic layer and substantially eliminates “extra” atoms originally included in the molecular precursor. In its most pure form, ALD involves the adsorption and reaction of each of the precursors in the complete absence of the other precursor or precursors of the reaction. In practice in any process it is difficult to avoid some direct reaction of the different precursors leading to a small amount of chemical vapor deposition reaction. The goal of any process claiming to perform ALD is to obtain device performance and attributes commensurate with an ALD process while recognizing that a small amount of CVD reaction can be tolerated.
In ALD applications, typically two molecular precursors are introduced into the ALD reactor in separate stages. For example, a metal precursor molecule, MLx, comprises a metal element, M that is bonded to an atomic or molecular ligand, L. For example, M could be, but would not be restricted to, Al, W, Ta, Si, Zn, etc. The metal precursor reacts with the substrate when the substrate surface is prepared to react directly with the molecular precursor. For example, the substrate surface typically is prepared to include hydrogen-containing ligands, AH or the like, that are reactive with the metal precursor. Sulfur (S), oxygen (O), and Nitrogen (N) are some typical A species. The gaseous precursor molecule effectively reacts with all of the ligands on the substrate surface, resulting in deposition of a single atomic layer of the metal:substrate−AH+MLx→substrate−AMLx-1+HL  (1)where HL is a reaction by-product. During the reaction, the initial surface ligands, AH, are consumed, and the surface becomes covered with AMLx-1 ligands, which cannot further react with metal precursor MLx. Therefore, the reaction self-terminates when all of the initial AH ligands on the surface are replaced with AMLx-1 species. The reaction stage is typically followed by an inert-gas purge stage that eliminates the excess metal precursor and the HL by-product species from the chamber prior to the separate introduction of the other precursor.
A second molecular precursor then is used to restore the surface reactivity of the substrate towards the metal precursor. This is done, for example, by removing the L ligands and re-depositing AH ligands. In this case, the second precursor typically comprises the desired (usually nonmetallic) element A (i.e., O, N, S), and hydrogen (i.e., H2O, NH3, H2S). The next reaction is as follows:substrate−A−ML+AHy→substrate−A−M−AH+HL  (2)
This converts the surface back to its AH-covered state. (Here, for the sake of simplicity, the chemical reactions are not balanced.) The desired additional element, A, is incorporated into the film and the undesired ligands, L, are eliminated as volatile by-products. Once again, the reaction consumes the reactive sites (this time, the L terminated sites) and self-terminates when the reactive sites on the substrate are entirely depleted. The second molecular precursor then is removed from the deposition chamber by flowing inert purge-gas in a second purge stage.
In summary, then, an ALD process requires alternating in sequence the flux of chemicals to the substrate. The representative ALD process, as discussed above, is a cycle having four different operational stages:                1. MLx reaction;        2. MLx purge;        3. AHy reaction; and        4. AHy purge, and then back to stage 1.        
This repeated sequence of alternating surface reactions and precursor-removal that restores the substrate surface to its initial reactive state, with intervening purge operations, is a typical ALD deposition cycle. A key feature of ALD operation is the restoration of the substrate to its initial surface chemistry condition. Using this repeated set of steps, a film can be layered onto the substrate in equal metered layers that are all identical in chemical kinetics, deposition per cycle, composition, and thickness.
Self-saturating surface reactions make ALD insensitive to transport non-uniformities, which might otherwise impair surface uniformity, due either to engineering tolerances and the limitations of the flow process or related to surface topography (that is, deposition into three dimensional, high aspect ratio structures). As a general rule, a non-uniform flux of chemicals in a reactive process generally results in different completion times at different areas. However, with ALD, each of the reactions is allowed to complete on the entire substrate surface. Thus, differences in completion kinetics impose no penalty on uniformity. This is because the areas that are first to complete the reaction self-terminate the reaction; other areas are able to continue until the full treated surface undergoes the intended reaction.
Typically, an ALD process deposits about 0.1-0.2 nm of a film in a single ALD cycle (with numbered steps 1 through 4 as listed earlier). A useful and economically feasible cycle time must be achieved in order to provide a uniform film thickness in a range of from about 3 nm to 300 nm for many or most semiconductor applications, and even thicker films for other applications. Industry throughput standards dictate that substrates be processed in 2 minutes to 3 minutes, which means that ALD cycle times must be in a range from about 0.6 seconds to about 6 seconds.
An ALD process must be able to execute this sequencing efficiently and reliably for many cycles in order to allow cost-effective coating of many substrates. In an effort to minimize the time that an ALD reaction needs to reach self-termination, at any given reaction temperature, one approach has been to maximize the flux of chemicals flowing into the ALD reactor, using a so-called “pulsing” process. In the pulsed ALD process, a substrate sits in a chamber and is exposed to the above sequence of gases by allowing a first gas to enter the chamber, followed by a pumping cycle to remove that gas, followed by the introduction of a second gas to the chamber, followed by a pumping cycle to remove the second gas. This sequence can be repeated at any frequency and variations in gas type and/or concentration. The net effect is that the entire chamber experiences a variation in gas composition with time, and thus this type of ALD can be referred to as time dependent ALD. The vast majority of existing ALD processes are time dependent ALD.
In order to maximize the flux of chemicals into the ALD reactor, it is advantageous to introduce the molecular precursors into the ALD reactor with minimum dilution of inert gas and at high pressures. However, these measures work against the need to achieve short cycle times and the rapid removal of these molecular precursors from the ALD reactor. Rapid removal in turn dictates that gas residence time in the ALD reactor be minimized.
Existing ALD approaches have been compromised with the trade-off between the need to shorten reaction times and improve chemical utilization efficiency, and on the other hand, the need to minimize purge-gas residence and chemical removal times. One approach to overcome the inherent limitations of time dependent ALD systems is to provide each reactant gas continuously and to move the substrate through each gas in succession. In these systems a relatively constant gas composition exists, but is located to specific areas or spaces of the processing system. Therefore, these systems will be referred to as spatially dependent ALD systems.
For example, U.S. Pat. No. 6,821,563 entitled “GAS DISTRIBUTION SYSTEM FOR CYCLICAL LAYER DEPOSITION” to Yudovsky describes a spatially dependent ALD processing system, under vacuum, having separate gas ports for precursor and purge gases, alternating with vacuum pump ports between each gas port. Each gas port directs its stream of gas vertically downward toward a substrate. The separate gas flows are separated by walls or partitions, with vacuum pumps for evacuating gas on both sides of each gas stream. A lower portion of each partition extends close to the substrate, for example, about 0.5 mm or greater from the substrate surface. In this manner, the lower portions of the partitions are separated from the substrate surface by a distance sufficient to allow the gas streams to flow around the lower portions toward the vacuum ports after the gas streams react with the substrate surface.
A rotary turntable or other transport device is provided for holding one or more substrate wafers. With this arrangement, the substrate is shuttled beneath the different gas streams, effecting ALD deposition thereby. In one embodiment, the substrate is moved in a linear path through a chamber, in which the substrate is passed back and forth a number of times.
Another approach using continuous gas flow spatially dependent ALD is shown in U.S. Pat. No. 4,413,022 entitled “METHOD FOR PERFORMING GROWTH OF COMPOUND THIN FILMS” to Suntola et al. A gas flow array is provided with alternating source gas openings, carrier gas openings, and vacuum exhaust openings. Reciprocating motion of the substrate over the array effects ALD deposition, again, without the need for pulsed operation. In the embodiment of FIGS. 13 and 14, in particular, sequential interactions between a substrate surface and reactive vapors are made by a reciprocating motion of the substrate over a fixed array of source openings. Diffusion barriers are formed by a carrier gas opening between exhaust openings. Suntola et al. state that operation with such an embodiment is possible even at atmospheric pressure, although little or no details of the process, or examples, are provided.
While processes such as those described in the '563 Yudovsky and '022 Suntola et al. patents may avoid some of the difficulties inherent to pulsed gas approaches, these processes have other drawbacks. For example, it would be very difficult to maintain a uniform vacuum at different points in an array and to maintain synchronous gas flow and vacuum at complementary pressures, thus compromising the uniformity of gas flux that is provided to the substrate surface. Neither the gas flow delivery unit of the '563 Yudovsky patent nor the gas flow array of the '022 Suntola et al. patent can be used in closer proximity to the substrate than about 0.5 mm.
U.S. Patent Publication No. 2005/0084610 to Selitser discloses an atmospheric pressure atomic layer chemical vapor deposition process. Selitser states that extraordinary increases in reaction rates are obtained by changing the operating pressure to atmospheric pressure, which will involve orders of magnitude increase in the concentration of reactants, with consequent enhancement of surface reactant rates. The embodiments of Selitser involve separate chambers for each stage of the process, although FIG. 10 shows an embodiment in which chamber walls are removed. A series of separated injectors are spaced around a rotating circular substrate holder track. Each injector incorporates independently operated reactant, purging, and exhaust gas manifolds and controls and acts as one complete mono-layer deposition and reactant purge cycle for each substrate as is passes there under in the process. Little or no specific details of the gas injectors or manifolds are described by Selitser, although it is stated that spacing of the injectors is selected so that cross-contamination from adjacent injectors is prevented by purging gas flows and exhaust manifolds incorporated in each injector.
A spatially dependent ALD process can be accomplished with other apparatus or systems described in more detail in commonly assigned U.S. application Ser. No. 11/392,007, U.S. application Ser. No. 11/392,006, U.S. application Ser. No. 11/620,744, and U.S. application Ser. No. 11/620,740. All these identified applications are hereby incorporated by reference in their entirety. These systems attempt to overcome one of the difficult aspects of a spatial ALD system, which is undesired intermixing of the continuously flowing mutually reactive gases. In particular, U.S. application Ser. No. 11/392,007 employs a novel transverse flow pattern to prevent intermixing, while U.S. application Ser. No. 11/620,744 and U.S. application Ser. No. 11/620,740 employ a coating head partially levitated by the pressure of the reactive gases of the process to accomplish improved gas separation.
Scientists have theorized in recent years that amorphous films are more robust than poly-crystalline or crystalline films. The advantages over polycrystalline films are a decreased surface roughness and an absence of grain boundaries that act as both charge trapping sites and diffusion pathways. While single crystalline films would generally have the highest mobilities, the deposition of a film that is single crystalline over a large area is very difficult, especially if flexibility in the choice of substrate is desired. Amorphous zinc oxide-based semiconductors are especially advantaged because the penalty in carrier mobility going from the crystalline to the amorphous state is much less compared to crystalline and amorphous silicon. This is due to the fact that the conduction band of ZnO is comprised of s-type wave functions that are spherically symmetric, whereas p-type wave functions make up the conduction band of Si.
Indium-doped zinc-oxide films and indium zinc oxides (IZOs) have received attention both as conductive transparent oxides and as transparent semiconducting materials for use in devices such as thin film transistors (TFTs). As noted above, these types of films are typically made by vacuum processes such as rf magnetron sputtering. Song et al. reported in 2007 on amorphous indium zinc oxide TFTs formed at room temperature by rf magnetron sputtering (Song et al., Applied Physics Letters 90, 022106 (2007)). In order to control the carrier concentration of the active channel, the partial pressure of oxygen in the sputtering chamber had to be controlled.
Studies on indium-doped zinc-oxide films formed by chemical spray pyrolosis were done by Joseph, et al. (Joseph et al., Bull. Mater. Sci. 28, 5, (2005)). Joseph, et al. were able to form low resistivity IZO films which, however, were polycrystalline in nature. Additionally, the spray pyrolysis technique used requires substrate temperatures of 723 K or approximately 450° C.
A common approach to chemically modifying the nature of a semiconductor is to include in the matrix of the semiconductor other atoms or molecules which vary the electrical properties of the semiconductor. These additional atoms typically operate by accepting or donating mobile charge to the system. In the case of a semiconductor like zinc oxide, acceptor dopants can be used to trap electrons, thus driving the semiconductor to have increased hole concentration and toward a p-type semiconductor. Alternatively, donor dopants tend to release electrons and in a semiconductor like zinc oxide this can produce high numbers of conduction band electrons, a desirable effect for applications requiring conductivity. Indium is a donor dopant for a zinc-oxide-based material.
In some instances, incorporating both donor and acceptor dopants into a zinc-oxide-based material can increase the control one has in selecting carrier type and concentration. Chen et al. found that for zinc oxide films co-doped with indium and nitrogen they could vary the films from p-type to n-type by controlling the substrate temperature (Chen, et al., Applied Physics Letters 89, 252113 (2006)). Chen et al. used direct current (DC) reactive magnetron sputtering for their experiments, varying the substrate temperatures between 440° C. and 600° C.
It is, thus, known that indium incorporation in zinc-oxide-based films can provide the benefit of decreased coating crystallinity and increased conductivity. There remains a need to provide a useful and cost effect process for producing indium-doped a zinc-oxide-based films.